1. Field of the Invention
The present invention relates generally to telecommunications networks and, more particularly, to a system and method for the signaling, routing and other manipulation of voice and data calls within the public switched telephone network.
2. Related Art
Telecommunication networks were originally designed to connect one device, such as a telephone, to another device using switching services. Circuit-switched networks provide a dedicated, fixed amount of capacity (a “circuit”) between two devices for the entire duration of a transmission session.
Originally, a circuit was created manually, i.e., by a direct connection from a calling party to a human operator (a “ring down”) along with human cross-connection by the operator to a called party.
More recently, a circuit is set up between an originating switch and a destination switch using a process known as signaling. Signaling sets up, monitors, and releases connections in a circuit-switched system. Different signaling methods have been devised. Telephone systems formerly used in-band signaling to set up and “tear down” calls. Signals of an in-band signaling system are passed through the same channels as the information being transmitted. Early electromechanical switches used analog or multi-frequency (MF) in-band signaling. Thereafter, conventional residential telephones used in-band dual-tone multiple frequency (DTMF) signaling to connect to an end office switch. Here, the same wires (and frequencies on the wires) were used to dial a number (using pulses or tones), as are used to transmit voice information. However, in-band signaling permitted unscrupulous callers to use a device such as a whistle to mimic signaling sounds to commit fraud (e.g., to prematurely discontinue billing by an interexchange carrier (IXC), also known as long distance telephone company).
More recently, to prevent such fraud, out-of-band signaling systems were introduced that use, for example, a packet network for signaling that is separate from the circuit switched network used for carrying information. For example, integrated services digital network (ISDN) uses a separate channel, a data (D) channel, to pass signaling information out-of-band. Common Channel Interoffice Signaling (CCIS) is a network architecture for out-of-band signaling. A popular version of CCIS signaling is Signaling System 7 (SS7). SS7 is an internationally recognized system optimized for use in digital telecommunications networks.
SS7 out-of-band signaling provided additional benefits beyond fraud prevention. For example, out-of-band signaling eased quick adoption of advanced features (e.g., caller-id) by permitting modifications to the separate signaling network. In addition, the SS7 network enabled long distance “Equal Access” (i.e., 1+ dialing for access to any long distance carrier) as required under the terms of the modified final judgment (MFJ) requiring divestiture of the Regional Bell Operating Companies (RBOCs) from their parent company, AT&T.
While SS7 and other out-of-band signaling systems have advantages over in-band systems, they still have deficiencies. For example, the SS7 network is still more like X.25 rather than a broadband network. Also, SS7 is a limited protocol in that it only addresses setup, teardown, and monitoring of calls.
An SS7 network includes a variety of components. Service Switch Points (SSPs) are telephone offices which are directly connected to an SS7 network. All calls must originate in or be routed through an SSP switch. Calls are passed through connections between SSPs within the telecommunications network. A Signal Transfer Point (STP) is a component which passes signals between SSPs, other STPs, and Service Control Points (SCPs) for processing. An STP is a special application packet switch which operates to pass signaling information. Two STPs may be used together to provide redundancy.
An SCP is a special application computer which maintains information in a database required by users of the network. SCP databases may include, for example, a credit card database for verifying charge information or an “800” database for processing toll-free calls. The components in the SS7 network are connected by links. Typically, links between SSPs and STPs can be, for example, A, B, C, D, E or F links. Typically, redundant links are also used for connecting an SSP and its corresponding STPs. Customer premises equipment (CPE), such as a telephone, are connected to an SSP or an end office (EO).
To initiate a call in an SS7 telecommunications network, a calling party using a telephone connected to an originating end office (EO) switch, dials a telephone number of a called party. The telephone number is passed from the telephone to the SSP at the originating end office (referred to as the “ingress EO”) of the calling party's local exchange carrier (LEC). A LEC is commonly referred to as a local telephone company. First, the SSP will process triggers and internal route rules based on satisfaction of certain criteria. Second, the SSP will initiate further signals to another EO or access tandem (AT), for example, if necessary. The signaling information can be passed from the SSP to STPs, which route the signals for communication between the ingress EO and the terminating end office, or egress EO. The egress EO has a port designated by the telephone number of the called party. The call is set up as a direct connection between the EOs through tandem switches if no direct trunking exists or if direct thinking is full. If the call is a long distance call, i.e., between a calling party and a called party located in different local access transport areas (LATAs), then the call is connected through an inter exchange carrier (BCC) switch of any of a number of long distance companies. Such a long distance call is commonly referred to as an inter-LATA call. LECs and IXCs are collectively referred to as the public switched telephone network (PSTN).
Emergence of a competitive LEC (CLEC) was facilitated by passage of the Telecommunications Act of 1996, which authorized competition in the local phone service market. Traditional LECs or RBOCs are now also known as incumbent LECs (ILECs). Thus, CLECs compete with ILECs in providing local exchange services. A large cost associated with setting up and operating a CLEC is the equipment needed to circuit switch data and voice calls.
Since the LECs handle both voice and data communications, large amounts of information are communicated. Bandwidth concerns are always present. The PSTN still has deficiencies, particularly with regard to data communications, for such problems as network congestion and bottlenecks.
The PSTN is ill-equipped to handle the integration of data and voice communications. Today, data and voice calls are sent through the same network. Data communications are presently layered on top of voice switching.
Circuit switching is the process of setting up and keeping a circuit open between two or more users, such that the users have exclusive and full use of the circuit until the connection is released. Packet switching is like circuit switching in that it can also switch information between users. Unlike circuit switching, packet switching does not leave a circuit open on a dedicated basis. Packet switching has conventionally been a data switching technique. Packet switching separates a communication into pieces called packets. A packet can contain addressing information, such as, for example, a destination address. In packet switching, the addresses of a packet are read by a switch and the packet is then routed down a path toward a switch associated with the destination address. Different packets can take diverse paths to reach the eventual destination. Typically, in the last switching office before the packets reach the destination user, the packets can be assembled and sequenced.
A channel, also known as a circuit, is a 64 (Kbps) building block of T1 series. A circuit is derived from the digitization and coding of analog signals. Digitization involves taking 8000 samples per second (i.e., twice the highest voice frequency of 4,000 Hz) for voice traffic. When coded in 8 bit words a 64 Kbps building block is yielded. This circuit is termed a Level 0 Signal and is represented by DS-0 (Digital Signal at Level 0). Combining 24 of these channels into a serial bit stream using time division multiplexing (TDM) is performed on a frame-by-frame basis. A frame is a sample of all 24 channels (i.e., the multiplicative product of 24 and 8 bits is 192 bits) plus a synchronization bit called a framing bit, which yields a block of 193 bits. Frames are transmitted at a rate of 8,000 per second (corresponding to the sampling rate), thus creating a 1.544 Mbps (i.e., the product of 8,000 and 193 is 1.544 Mbps) transmission rate, which is the standard T1 rate. This rate is termed DS-1.
Queuing refers to the act of stacking or holding calls to be handled by a specific person, trunk or trunk group. Queuing theory deals with the study of the behavior of a system that uses queuing, such as a telephone system. Queuing is very important to the design of packet networks where speed of transmission more than offsets the delay of waiting for a transmission facility to become available.
Telephone call traffic is measured in terms of centi call seconds (CCS) (i.e., one hundred call seconds of telephone conversations). One hour of calling traffic, also known as an Erlang (named after a queuing theory engineer), is equal to 36 CCS (i.e., the product of 60 minutes per hour and 60 seconds per minute divided by 100, the theoretical limit of a trunk). An Erlang is used to forecast trunking and TDM switching matrix capacity. A “non blocking” matrix (i.e., the same number of lines and trunks) can theoretically switch 36 CCS of traffic. Numerically, traffic on a trunk group, when measured in Erlangs, is equal to the average number of trunks in use during the hour in question. For example, if a group of trunks carries 20.25 Erlangs during an hour, a little more than 20 trunks were busy.
At times of high data traffic, the internal CCS of call traffic of the tandem and egress switches climbs, resulting in such problems as network blocking and busy signals. Data calls traditionally pass through tandem and egress switches before being switched to a Wide Area Network (WAN) access device. The tandem and egress switches have become bottlenecks.
Growth of the Internet has led to increased data communications traffic that has exacerbated the problem. Corporations that provide remote modem access to data networks provide dial-up and direct connections. One important example of such corporations are Internet Service Providers (ISPs) provide dial-up and direct connection access to Internet subscribers. Dial-up access is based on transmission using the serial line interface protocol (SLIP) or point-to-point protocol (PPP) to the ISP's network access device. An ISP's network access device can include a communications server. A communications server represents one of several devices connected to a local area network (LAN) or wide area network (WAN). A network router can be connected to the LAN. A network router can be, for example, a computer running routing software, or a dedicated routing device. The router's serial port is used to provide a high-speed communications connection from the ISP to an Internet network service provider (NSP).
Many ISPs are small, start-up companies that face challenges in obtaining the startup capital required to fund large capital expenditures required to purchase the data termination and protocol conversion equipment, including routers, communications servers, and racks filled with modems. ISPs must also expend significant sums of money to the ILEC for large numbers of access lines required to pass data calls through tandem and egress switches before being switched to WAN access devices. ISPs must pass on these costs to their subscribers.
Similarly, a business entity must also invest substantial capital to purchase communications equipment, when, for example, the entity needs to provide employees remote access to a private data network.
The attributes of modem or Internet-type data traffic are very different from those of voice traffic. First, the traffic is qualitatively different. The duration of data traffic (e.g., 20 minutes, 12 hours, or more) is typically longer than voice traffic (e.g., 3 minutes) and therefore requires different queuing theory. Ironically, a data call often does not even need access to the line all the time since an Internet call can contain “bursty traffic”, i.e., intermittent bursts of upstream and downstream traffic. Because voice and modem traffic are structurally different, the probability distribution must be adjusted accordingly. The statistical distribution for voice calls is an “exponential distribution,” i.e., most calls are 3 minutes or less in duration, and there is a rapidly decreasing number of calls lasting longer than 3 minutes. Data calls (e.g., modem, fax, internet, etc.) have a mean holding time on the order of 20 minutes, and the distribution of holding times instead of having an exponential distribution, has a “power law distribution,” meaning it is not extraordinary to encounter calls of very long duration such as, e.g., 12 hours, a day, or even longer.
Second, modem internet traffic is also quantitatively different from voice traffic. The Internet modem traffic generates much higher loads. Residential lines have been engineered expecting to generate loads of 3 or 4 CCS, and business lines, 5 or 6 CCS. If the same customer begins using the same line for Internet traffic, the load can easily double or triple.
Today, the public network is optimized for voice. However, modem traffic has overtaken voice in the local exchange. Queuing theory has not been adjusted for this occurrence, resulting in public network dysfunction. For example, growth in popularity of fixed rate, unlimited access services from ISPs has excessively burdened the PSTN circuit-switch infrastructure. Each unlimited access connection can tie up a dedicated circuit through a tandem switch and/or an egress end office (EO) switch. What is needed then is an improved system for handling data communications, which would allow data to bypass the local exchange's egress switches and the associated costs from local telephone companies.